A protective element such as a protective diode is generally disposed in the semiconductor substrate in which the MOSFET is formed. In a MOSFET having a structure in which the semiconductor substrate is also to function as a drain region (hereinafter referred to as a "vertical MOSFET"), however, it has been found that when a protective diode is provided, a thyristor operation occurs due to the presence of a parasitic transistor, causing permanent breakdown of the vertical MOSFET. This is a critical problem in practice.
To solve this problem, it has previously been proposed to provide a structure in which a protective element of this kind is disposed on an insulating film isolated from the MOSFET substrate (e.g. see U.S. Pat. No. 4,492,974). In this prior proposal, the protective element and the gate electrode of the MOSFET were formed of polycrystalline silicon and electrically connected to each other by a metallic wiring layer such as aluminum. This resulted in another problem of increased chip area.